In the production of hydrocarbons from low permeability formations, fracture stimulation is often employed to enhance productivity and improve deliverability of the production fluid. This is typically accomplished through hydraulic fracturing, which involves the introduction of a gel or other high viscosity fluid into the formation of interest under sufficiently high pressure to create fissures in the formation. At the conclusion of the fracturing process the viscosity of the hydraulic fluid is reduced, commonly by the introduction of a gel breaker or by the action of a gel breaker originally included in the fluid. When the high pressure is released the formation fluid flows through the newly created fissures at an increased rate. In order to maintain the fissures in an open condition after removal of the hydraulic fracturing fluid, propping agents such as sand are included in the hydraulic fluid and are carried with it into the newly formed fissures. When the hydraulic fluid is removed the sand remains, holding the fissures open.
Although such hydraulic fracturing procedures are well known in the industry, it is nevertheless often difficult to concentrate the process in the particular producing zone of interest due to the tendency of the fracturing fluid to enter surrounding nonproducing layers. To afford better control of the flow of the fracturing fluid the zone to be fractured ideally should be at a lower pressure than the surrounding confining layers, which would cause the hydraulic fluid to preferentially seek and remain in the zone of interest. Prior to the present invention, no practical way to achieve this condition has been known.